Lawyer of Ping reported missing
Iga Iga's wife reported that men had searched the family home in the presence of the couple's children, Nambo added.
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Eric Iga Iga, one of two lawyers who represented Ping in Constitutional Court in September when he sought to challenge Bongo's victory, vanished without explanation on Thursday, Iga Iga's associates said.
"He told me on the phone that a suspicious car was waiting at his house and he was trying to find out more," John Nambo, Ping's chief of staff, told AFP.
Jean-Remy Bantsantsa, the other lawyer who represented Ping, said of the disappearance: "We have been round to all the police stations and the gendarmerie posts."
The Constitutional Court in September upheld Bongo's victory, but Ping nevertheless declared himself "president-elect" when he returned to Gabon after spending time in Paris, Brussels and Washington in the aftermath of the election.
International powers have largely recognised the result while calling for political dialogue in Gabon.
On Monday, a European Union election observer mission is due to present its report on the August 27 presidential election.
At least three people were killed in post-electoral violence, according to the government, while the opposition says at least 26 died and several others are still missing.
Ping, a former top official at the African Union, has said the final report of the EU's observer mission would "reveal the truth of the deception".
An initial report by the EU observers noted "evident anomalies" in the results, notably in Bongo's stronghold of Haut-Ogooue.
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